Inverting an "S" to a letter other than "S" is often hard, because it normally looks the same in both directions. ;) The only way I found until now to fix this is to make one of the squiggles look unimportant and small in one of the two cases, and make it look more relevant in the other. (Also compare to the Silke ambigram.)
Inverting the "RI" in the middle is surprisingly easy. A similar trick for the lowercase "r" character can be found in the Martina ambigram.
More of my ambigrams are here.